1. Field of the Invention
The present invention generally relates to the computer-assisted design of complex systems and, more particularly, to the design and analysis of waterborne vessels, especially for specific fuel consumption performance.
2. Description of the Prior Art
Many areas of human endeavor involve the design of complex systems which may involve application of the latest engineering knowledge in a wide variety of technologies. Naval architecture is a particular field in which many engineering disciplines must be employed at a very high level to accomplish designs having desired performance. In the design of a new ship, for example, the ideal design strategy is to start with the closest existing design, determine changes dictated by desired differences in performance or intended use and to then have those changes reconciled by each of a succession of groups of engineers highly skilled in a limited area of ship design; each group further implementing changes to reconcile not only changes from the original design but changes implemented by preceding groups in response to those changes. Since this process is considered likely to converge (or rapidly reveal divergence when an integrated solution is not possible) it has come to be known as a design spiral, schematically illustrated in FIG. 1.
As shown therein, the purposes of the new ship are first, for example, accommodated by definition of displacement and general dimensions and allocation of the large volumes of the vessel. The design is then evaluated for machinery requirements such as motive power, auxiliary power, hydraulics for control, on board heavy equipment, etc. and the dimensions of required machinery and other required spaces such as stack volumes are determined and incorporated into the design. Then the design is passed to another group which determines the required length of the vessel based on the initially designated volumes and the volumes required for machinery. Then the design is passed to a group to determine other hull parameters such as draft, beam, freeboard given the intended use and anticipated sea conditions and the design parameters established by the previous design groups.
Next, the hull shape is determined to provide minimum drag and other performance requirements within the basic hull dimension specifications. Then, another group determines the displacement in accordance with the preceding design work and specifications so far developed, such as inclusion of machinery weight. At this point, hull performance can be evaluated and propulsion requirements defined for desired cruising and top speeds. Then, endurance requirements can be evaluated and accommodated by determination of fuel requirements and the volume of the vessel which will be required to contain a sufficient amount of fuel. Then weights, including fuel can be evaluated and previously determined displacement and volume allocations can be refined. Finally, stability is evaluated and previous specifications refined to improve the performance and insure safety in anticipated sea conditions.
At this point, however, volume allocation may have been changed to the point that a further cycle of design refinements might ideally be called for and the cycle could, in theory, be repeated until no further design changes were specified by any group. This circumstance corresponds to the inner circle terminating the design spiral of FIG. 1 when all facets of the design converge.
As a practical matter, however, the cost of this analysis and design refinement is so great and time consuming that the design will be considered complete well before such a convergence occurs. For this reason, designs have, in the past, often been far less than optimal and performance of vessels, when constructed, has often been disappointing and below design specifications. In some cases, the resulting vessel, when constructed, may be unsuitable for the service intended or the performance so marginal that customer or user acceptance will be impaired; having adverse effects on profitability of the vessel or other aspects of its use. In naval vessels in particular, various aspects of the performance of a vessel may seriously impact the ability of the crew to carry out its assigned mission.
Therefore, in order to more fully refine ship designs, a family of computer implemented ship design programs have recently been developed for use in the early design phases of surface ships. These computer programs are proprietary to the United States Navy and are collectively referred to as the Advanced Surface Ship Evaluation Tool (ASSET). Currently, a separate program is used for each type of ship (e.g. hydrofoils, tankers, etc.) for which a design is required. An exemplary set of procedures is shown in FIG. 2 as a circulating flow chart generally implementing a design spiral, as described above, each "step" depicted, itself, being a program for that portion of the design and referred to as a "module". As a final step in the process, the performance of the design is evaluated and a determination is made as to whether or not convergence can be considered to have been achieved.
Even with very substantial computing power, however, an iteration of one loop of this system can be very expensive in terms of processor time. Particularly when numerous technologies are available for use in a portion of the design, such as for power systems, as is presently the case, and these technologies may have a major impact on many aspects of the design, such as performance, weights, volumes and, especially, endurance, it was found that performing an entire circuit of the loop to evaluate many possible choices for power systems was impractical and presented a major obstacle to the efficacy of the entire computer implemented design procedure. On the other hand, since engine selection impacts many features of the design, especially from the standpoint of space allocation for fuel tankage, results must be obtained from many modules of the asset program to determine the appropriateness of a particular engine choice and no alternative existed for obtaining such information without traversing the design refinement loop. Further, diverse technologies may require different forms of analysis for simulation and have different requirements (e.g. stacks, gear boxes, auxiliaries) which may force alternative courses in design; potentially causing design divergence.